This invention relates to reinforced carbon bodies. More particularly, it relates to reinforced carbon bodies composed of a matrix of fibrous carbon material bonded together and substantially coated by a deposited pyrolytic material and having the residue of a carbonized filler material deposited on said pyrolytic material, preferably to interconnect said pyrolytic material. The articles comprise an underlying matrix or network of fibrous carbon material which forms the skelton of the carbon body and may be produced by infiltrating the matrix with a pyrolytic material in a manner to depose the pyrolytic material on the fibrous carbon material and bond together the fibrous carbon material to form a bonded body, subsequently impregnating said bonded body with a carbonizable filler, curing and carbonizing the filler, and, if desired, repeating the impregnating, curing and carbonizing steps. The final product may be graphitized. Such bodies usually contain from about 10 percent to about 65 percent by volume of reinforcing carbon fibers, preferably about 45 percent, and from about 35 percent to about 90 percent by volume of carbonaceous material, preferably about 55 percent. As used hereafter, the term "carbon" includes both ungraphitized and graphitized carbon. Thus, a reinforced carbon body can comprise either graphitized, partially graphitized or ungraphitized reinforcing carbon fibers or a mixture thereof and either graphitized, partially graphitized or ungraphitized carbon filler.
Reinforced carbon bodies of the present invention can also include material other than carbon to modify the properties in various ways. For example, U.S. Pat. No. 3,672,936 describes the use of boron and boron compounds to improve several of the properties.
The specific reinforced carbon bodies with which the present invention is particularly concerned are those which are subjected to circumferencial stress. A prime example of such carbon body is a friction disc for use in disc brakes. Such discs are essentially annular in shape, having outer and inner perimeters, at least one perimeter of each disc being provided with a friction-bearing surface. When contact occurs with the friction-bearing surface, the mechanical energy of the rotating portion of the brake is converted to heat. Because of carbon strength, density, heat capacity, thermal conductivity, co-efficient of friction and stability to its sublimation temperature (about 3600.degree. C.), carbon has been proposed for use in disc brakes, particularly where weight is a major consideration (such as in aircraft).
One method of producing articles of this type is to coat sheets of graphite cloth with a suitable binder, stack the sheets and heat the stacked sheets to carbonize the binder. When applying the coating of binder to the graphite cloth sheet, it is extremely difficult, if not impossible, to avoid variations in the thickness of the binder layer on the sheet. Consequently, as the sheets are stacked, there will be a variation of the binder thickness between the graphite cloth sheets. As the stacks of graphite cloth sheets are heated in order to first cure and then carbonize the binder, the binder will expand within the stacks at an uneven rate due to the variation of the binder thickness. At the carbonizing temperature the binder, as it is converted into carbon, contracts, and this contraction will also occur at a non-uniform rate due to the variation in thickness of the original binder layer. The resulting body has set up within it internal stresses due to the uneven rate of expansion and contraction of the binder which eventually will lead to cracks within the body when subjected to conditions of stress. Accordingly, when forming laminated carbon or graphite articles by this process, it is sometimes necessary to restrict the thickness of the article in order to hold the uneven expansion and contraction of the binder caused by variations in binder thickness to a minimum.
Another method of producing articles of this type is to pyrolytically deposit carbon by known vapor deposition techniques on fibrous carbon material to produce a reinforced carbon product. However, this method generally produces a product that is inconsistent in physical properties from batch to batch. It is postulated that the individual carbon crystallites arrange themselves in a planar manner when being deposited on the fibrous carbon material, and, as it is attempted to densify the articles, the crystallites, while filling the voids between fibers, also seal off both the deeper voids and the surface, leaving the inner portion impermeable to further deposition within the article. Thus, such deposition alone produces an article having good surface characteristics but, when shaped or machined, is frequently found not to have the desired strength and wear characteristics.
As used herein, the following terms shall have the following meaning:
(a) Fibrous Carbon Material is produced by the heat treating of both natural and synthetic fibers of materials such as wool, rayon, polyacrolynitrile and pitch at high temperatures.
(b) Pyrolytic Material refers to the material which is deposited on a substrate by the thermal pyrolysis of a gas.
(c) Pyrolytic Carbon refers to the carbon which is deposited on a substrate by the thermal pyrolysis of a carbon containing gas.
(d) Carbonizable Filler Material refers to char-producing materials such as petroleum pitch or polymeric plastics or resins that have ablative characteristics. For example, included in the latter group are: the phenolics and modified phenolics, furanes, polyimides, melamines, ureas, and epoxies. The polymer of liquid furfuryl alcohol has been found to have penetrating and flow characteristics which are particularly suited to use in the present invention.
(e) Density refers to bulk or apparent density.